I. Field
The present disclosure relates generally to wireless communication, and more specifically to techniques for estimating the propagation channel in a wireless communication system.
II. Background
Currently there are numerous wireless communication systems which provide various types of communication services such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources. Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
OFDM effectively partitions the overall system bandwidth into a number of (N) orthogonal subbands. These subbands are also referred to as tones, frequency bins, and frequency subchannels. With OFDM, each subband is associated with a respective subcarrier upon which data may be modulated. Each subband may thus be viewed as an independent transmission channel that may be used to transmit data.
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver through a number of propagation paths. For an OFDM system, the N subbands may experience frequency selective fading due to the effects of fading and multipath.
An accurate estimate of the response of the wireless channel between the transmitter and the receiver is normally needed in order to effectively decode the transmit data on the available subbands. In OFDM systems, the propagation channel is estimated by sending several pilot tones in the frequency domain. The receiver extracts these pilot measurements in the frequency domain and performs an IFFT operation to get an estimate for the impulse response of the channel in the time domain. The length of this impulse response is normally limited to the length of the cyclic prefix of the OFDM symbol. Since these pilot measurements may be corrupted by noise at the receiver, there will be energy in all impulse response taps. However, not all the taps in the estimated impulse response correspond to an actual channel taps. Some of the taps will have energy due to noise only. One method to reduce to effect of noise in the impulse response taps is to identify the taps that are most likely due to noise and zero them out, thereby suppressing the noise effect from those taps. However, if the signal to noise ratio (SNR) is very low, there is a strong likelihood that noise-only taps will have more energy than actual channel taps. In this case, the noise-only taps will be picked instead of the actual channel taps, leading to an inaccurate estimate of the channel impulse response.
There is therefore a need in the art for techniques to more efficiently estimate the channel response in a multi-channel OFDM system.